Many inorganic materials are processed at high temperature to alter the composition and/or properties thereof. Such processes may involve shaping, such as reflowing, reforming, molding, pressing, cutting, grinding, other machining, and the like. Such processes may also involve crystal formation and growth, annealing, etc. For high purity materials, contamination during such high temperature processing may be a concern. This is especially true for high purity glass and crystalline materials for use in precision optical devices where very low metal concentration can significantly compromise the material performance. The high temperature adds to the concern because under those conditions, contaminants, especially metal ions, are usually more mobile than at lower temperature, thus their concentrations can be high in the treatment environment. Further, contamination at high temperature can occur much faster, even if the inorganic material is already densified. Once contaminated, further purification is usually difficult or impractical for those densified material; as a result parts with an unacceptable level of contamination has to be sacrificed, leading to lower production yield.
Such contamination during high temperature treatment is especially a pronounced problem for high purity synthetic silica glass for use in advanced lithographic devices. Synthetic silica glass finds use in optical elements in lithographic tools and other optical tools, such as laser generators and the like, operating at deep UV and vacuum UV regions. It is known that very low level of metal concentration, typically on the ppb level, are required for high performance lithography devices used in modern-day very-large-scale integrated circuits (VLSI) industry. The production and processing of the materials for such applications involve high temperature treatment such as reforming, annealing, and the like. The environment where such processing takes place, even if constructed with high purity materials, tends to have a higher than desirable sodium concentration. Thus control of metal contamination, especially sodium contamination, is a big problem.
The prior art does not provide a solution to this problem. The solution as presented herein can be extended to high temperature treatment of materials other than silica glass where metal contamination is an issue.